1. Field of the Invention
The present invention concerns a cathode of the type having a cathode head in which a surface emitter is arranged that emits electrons upon application of a heating voltage thereto.
2. Description of the Prior Art
A cathode of the above type in which the surface emitter has a rectangular footprint is known from DE 27 27 907 C2, for example. A surface emitter with a circular footprint is described in DE 199 14 739 C1. In the known surface emitters, a heating voltage is applied to the surface emitter during the operation of the x-ray tube, whereby electrons are emitted that are accelerated in the direction of an anode. X-ray radiation is generated in the surface of the anode upon impact of the electrons at the anode.
Such a surface emitter has a distinctly larger radiant surface usable for emission relative to the volume to be heated and in comparison to a filament emitter. The surface emitter therefore can be operated with a reduced working temperature relative to a filament emitter, so the service life of the cathode is increased.
The longer service life of a surface emitter due to the larger radiant surface (emission surface) requires a greater effort for cutoff of the emitted electron beam.
This beam cutoff by application of a negative voltage to the cathode head is necessary in many applications, in particular in applications with pulsed x-ray radiation. The more central regions of large-area surface emitters are geometrically farther removed from the electron accumulations generating the cutoff field at the cathode head, and thus can only be cut off by higher electron concentrations or higher field strengths. Higher field strengths, in turn, require larger minimum distances to be maintained to avoid arcing, as well as additional design costs.